Some 71 individuals have been charged in the US with Islamic State-related activities since March 2014 - 58 of them US citizens.

Fifty-six of the total charged were arrested this year alone, found the researchers.

Islamic State recruits in the American legal system ranged in age from 15 to 47, with an average age of 26.

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About 40% of them were converts to Islam, according to the researchers.

The study cites US authorities as recording that some 250 Americans have travelled or attempted to travel to Syria and Iraq to join the Islamic State.

The extremists group's US sympathisers ranged from single mothers to teenage girls, petty criminals and college students.

The study's authors, who monitored the recruiting in real time, wrote: "In one case the seemingly naive individual posted general questions about religion, to which ISIS supporters quickly responded in a calm and authoritative manner.

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"After a few weeks, the accounts of hardened ISIS supporters slowly introduced increasingly ardent views into the conversation.

"The new recruit was then invited to continue the conversion privately, often via Twitter's Direct Message feature or on other private messaging platforms."

The report also looked at how US jihadists meet and recruit in person, uncovering small groups of apparent Islamic State sympathisers in a number of cities, including in Texas and the Midwest.